1. Field of the Invention
This invention relates to pneumatic tires, particularly a pneumatic tire having an excellent bead portion durability. More particularly, it relates to a pneumatic tire capable of preventing pulling-out of a ply cord and occurrence of separation failure at an end position of a turnup portion of a carcass ply and also effectively controlling failure of a bead portion due to bending deformation of the bead portion around a rim flange under an action of a high loading to improve the bead portion durability. The invention is favorably applied to so-called heavy duty pneumatic radial tires used under conditions of a high load and a high internal pressure such as trucks, buses and the like.
2. Description of Related Art
In order to prevent pulling-out of a ply cord during the running of the pneumatic tire under loading, as shown in FIG. 1a, a carcass ply 100 is usually wound around a bead core 102 of a bead portion 101 from an inside of the tire toward an outside thereof in a widthwise direction and a turnup portion 103 thereof is embedded and fixed in rubber.
In the conventional tire having such a bead portion structure, however, a sidewall portion 104 is subjected to a large bending deformation during the running of the tire under loading, particularly under an action of a large loading and hence pulling force F is applied to the carcass ply 100 in a direction shown by an arrow in FIG. 1a, while a step difference in stiffness is caused on inside and outside of the tire in a radial direction thereof about an end position 105 of the turnup portion 103 of the carcass ply 100 and also stress concentration is repeatedly caused in the end position 105 and a neighborhood thereof. As a result, there is a problem that separation failure of the ply cord from rubber is apt to be caused and results in the occurrence of crack Cr in the bead portion as shown in FIG. 1b. 
As means for preventing such a separation failure to improve the bead portion durability, there are various proposals that it is useful to render a turnup portion 107 of a carcass ply 106 into a wrap part 109 winding on the bead core along its peripheral face as shown in FIG. 2a. 
However, when a reinforcing layer such as a wire chafer or the like is not arranged in the bead portion, a large bending deformation is created in a direction shown by an arrow A about a rim flange 110 as shown by a phantom line in FIG. 2b. As a result, crack is apt to be created in the bead portion of the tire contacting with the rim flange by repeating such a bending deformation and in the worst case, there is a risk of causing the bead portion failure by the growth of such a crack.
As means for improving the bead portion durability of the tire having the above bead portion structure, it is useful that at least one reinforcing layer 112 wound around a bead core 111 from an inside of the tire toward an outside thereof in a widthwise direction at a position outside the carcass ply 110 is arranged in the bead portion to control the bending deformation of the bead portion under the action of high loading applied to the tire as shown in FIG. 3a. 
As shown in FIG. 3b, however, stress concentrates in an end position 113 of the reinforcing layer 112 and there is caused a fear of creating troubles such as crack Cr of bead portion and the like in such a position 113. As a result, such a means can not effectively improve the bead portion durability.
It is, therefore, an object of the invention to provide a pneumatic tire capable of preventing pulling-out of a ply cord and occurrence of separation failure at an end position of a turnup portion of a carcass ply and also effectively controlling failure of a bead portion due to bending deformation of the bead portion around a rim flange under an action of a high loading to improve the bead portion durability.
According to a first aspect of the invention, there is the provision of a pneumatic tire comprising at least one carcass ply of steel cords constituted with a main body portion toroidally extending from a tread portion through a sidewall portion to a bead portion and a turnup portion wound around a bead core embedded in the bead portion, in which a wrap part wound on the bead core along its outer peripheral face is disposed in the turnup portion of the carcass ply and at least one wire chafer wound around the bead core from the main body portion of the carcass ply toward the turnup portion thereof in a widthwise direction is embedded in the bead portion, and a shortest distance of a start end of the wire chafer located at the side of the main body portion of the carcass ply as measured outward from a normal line drawn to an outer face of the bead portion at a first rim line position in the radial direction of the tire is located to be within a range of 15-25 mm and a terminal end of the wire chafer located at the side of the turnup portion of the carcass ply is arranged within a range defined between a vertical line drawn from an outermost end position of the bead core in the radial direction to the outer face of the bead portion and the above normal line.
The term xe2x80x9cfirst rim line positionxe2x80x9dused herein means an outermost end in the radial direction of that portion of an outer face of the tire which contacts with a rim when the tire is mounted onto an approved rim and applied under a maximum air pressure and a maximum load capacity.
In this case, the terms xe2x80x9cmaximum air pressurexe2x80x9d and xe2x80x9cmaximum load capacityxe2x80x9d used herein are defined according to xe2x80x9cYear Book of The Tire and Rim Association Inc.xe2x80x9d in USA, xe2x80x9cStandard Manual of The European Tire and Rim Technical Organizationxe2x80x9d in Europe or xe2x80x9cJATMA Year Bookxe2x80x9d in Japan.
And also, the term xe2x80x9cturnup portion of carcass plyxe2x80x9d used herein includes a case that the carcass ply is wound around the bead core from an outside of the tire toward an inside thereof in the widthwise direction in addition to a case that the carcass ply is wound around the bead core from an inside of the tire toward an outside thereof in the widthwise direction.
In addition, the carcass ply is a ply wherein many steel cords each having both cut ends are arranged substantially in the radial direction but also includes a ply wherein a single steel cord is folded at a position corresponding to the turnup portion of the carcass ply and continuously extended while detouring in a circumferential direction of the tire.
When it is required to more control the occurrence of separation failure at the terminal end position of the wire chafer, it is favorable to arrange at least one organic fiber chafer covering the terminal end of the wire chafer at the side of the turnup portion of the carcass ply, and in this case the organic fiber cords in the organic fiber chafer are preferable to be arranged at an angle of 15-75xc2x0 with respect to an arranging direction of the steel cord in the wire chafer.
When it is required to more control separation failure at the start end position of the wire chafer, it is favorable to arrange a cushion rubber layer between the main body portion of the carcass ply and the start end portion of the wire chafer, and in this case a rubber thickness of the cushion rubber layer at the start end position of the wire chafer is preferable to be 1.5-2.0 mm at a section in the widthwise direction of the tire.
According to a second aspect of the invention, there is the provision of a pneumatic tire comprising at least one carcass ply of steel cords constituted with a main body portion toroidally extending from a tread portion through a sidewall portion to a bead portion and a turnup portion wound around a bead core embedded in the bead portion, a part or whole of which turnup portion being a wrap part wound along a peripheral face of the bead core, in which the main body portion of the carcass ply has a convexly curved shape toward an outer face of the tire or a combined shape of the curve and straight line at a section in a widthwise direction of the tire when the tire is mounted onto an approved rim and inflated at a maximum air pressure under no load, and a straight line or a chord connecting both ends of an arc drawn by the main body portion of the carcass ply over a zone ranging from a turning point of the main body portion on the bead core to any point in a region corresponding to 15-35% of a length in the radial direction of the tire defined between a standard line passing through an axial center of the bead core and drawn in parallel to a rotating axis of the tire and an outermost side of the main body portion in the radial direction and separated from the standard line is inclined at an angle of 45-65 xc2x0with respect to an equatorial plane of the tire.
In a preferable embodiment of the second aspect of the invention, the main body portion of the carcass ply has an arc shape convexly extending toward the outer surface of the tire over a zone ranging from the turning point of the main body on the bead core to any point in a region corresponding to 17-25% of the length in the radial direction of the tire separated from the standard line.
In another preferable embodiment, a maximum length of a vertical line drawn from the arc to a chord connecting both ends of the arc is not less than 2 mm.
According to the second aspect of the invention, the occurrence of separation failure at the turnup end of the carcass ply can be prevented by wrapping the turnup portion of the carcass ply on the peripheral face of the bead core, while the shape of the main body portion of the carcass ply in the bead portion is regulated to increase lateral stiffness of the bead portion, whereby outward fall-down deformation of the bead portion in the widthwise direction during the running of the tire under loading or the like can effectively be controlled to sufficiently prevent the occurrence of fatigue breakage in the bead portion contacting with a rim flange or the like.